Adaptive filters operate on a signal supplied thereto in accordance with prescribed criterion to generate a desired output signal. Typically, the filters are operative to generate a transfer function (an inpulse response characteristic) in accordance with an algorithm which includes updating of the transfer function characteristic in response to an error signal. In this way the filter characteristic is optimized to yield the desired result. We have determined, however, that the adaptive filters tend to diverge from the optimum filter. characteristic when the supplied signal includes a prescribed class of energy. Specifically, during intervals that the supplied signal includes energy occupying only a portion of a frequency band of interest, for example, a signal frequency tone, multi-frequency tone or the like (hereinafter designated partial band energy) the transfer function characteristic diverges resulting in an undesirable condition of the filter. In digital adaptive filters, the transfer function (impulse pulse response) characteristic is indeterminate when partial band energy is received and the register storing a representation of the impulse response characteristic tends to overflow causing the filter to reset and start the adaptive process over again. This condition is undesirable and should be avoided.
In an echo canceler an adaptive filter is used to generate an echo path estimate which is updated in response to an error signal. Echos commonly occur because of imperfect coupling of incoming signals at 4-to-2wire junctions in communications systems. The echos typically result because of imperfect impedance matching to the 2-wire facility in the 4-to-2 wire junction causing the incoming signal to be partially reflected over an outgoing path to the source of incoming signals.
Self-adapting echo cancelers have been employed to mitigate the echoes by adjusting the transfer function (impulse response) characteristic of an adaptive filter to generate an estimate of the reflected signal or echo and, then, subtract if from the outgoing signal. The filter impulse response characteristic and, hence, the echo estimate is updated in response to the outgoing signal for more closely approximating the echo to be cancelled. Heretofore, the updating of the echo estimate has been inhibited when near end speech signals are being transmitted or when no significant far end energy is being received. However, the echo estimate was allowed to be updated when any significant far end energy was being received, whether it was speech, noise, single frequency tones, multifrequency tones or the like.
We have determined that allowing the canceler to update the echo estimate during intervals that the received far end signal includes partial energy results in an undesirable condition of the communications circuit including the canceler. Specifically, the canceler includes a self-adapting processor, i.e., adaptive filter, which can adjust to a large number of inpulse response characteristics in order to generate the echo path extimate which best approximates the echo. A problem, in addition to that of divergence, with allowing the processor to adjust the transfer function when partial band energy is being received is that although the impulse response characteristic arrived at is optimized for the frequency components of the partial band energy it may not be optimum for the remaining frequency components in the frequency band of interest, for example, the voice band. Indeed, the impulse response adjusted to at frequencies other than those in the partial band energy may be significantly different from the desired optimum adjustment which would be obtained when adjusting on a whole band signal, i.e., speech or Gaussian noise. Consequently, a so-called low return loss path is established in the communications circuit at frequencies other than those in the partial band energy. This low return loss can lead to oscillations in the communications circuit. These oscillations are extremely undesirable and must be avoided.